The invention relates to an X-ray diagnostic device with an X-ray converter with a luminescent fluoroscopic screen which latently stores the respective X-ray image and which can be activated to light up for image reproduction by an additional source of radiation, the device having a scanning device for the fluoroscopic screen, an optical system, a detector, and a video monitor. In X-ray diagnostics, such a fluoroscopic screen serves to record and reproduce an X-ray picture.
In German Offenlegungsschrift No. 29 28 244 is described such an X-ray diagnostic device in which an X-ray picture in a plate is first stored as a latent image, by means of a fluoroscopic screen of a luminescent substance which can be activated by visible light or infrared rays. The electron-hole pairs generated by the absorption of the X-rays in the film of luminescent material of the fluoroscopic screen are retained in a potential trap of the luminscent material so that the X-ray picture remains stored. The number of electron holes depends on the amount of absorbing radiation energy. It is only through scanning of the fluoroscopic screen picture element by picture element, as by an infrared laser beam, that these electron holes are raised to the conduction band, emitting visible light when they fall back to their original states. The stored X-ray image is made visible through activation with visible light or infrared rays, whereby the electrons stored in the potential trap of the luminescent substance are released, thus releasing the X-ray picture stored in the fluoroscopic screen in the form of fluoroscent light. The fluoroscent light is captured by a photodetector and converted into an electrical signal which is converted into a video image by a monitor.
This reference states that known fluoroscopic screens are of insufficient sensitivity. While the relative sensitivity can be improved by the selection of the luminescent material, it still is insufficient in many cases to produce X-ray pictures of adequate brightness because, for example, the optical coupling of the fluoroscopic screen to the detector results in further losses so that the noise component of the detector is not negligible.
The invention is directed to the problem of designing an X-ray diagnostic device of the kind mentioned so that the relative sensitivity is further improved and the output image of the fluoroscopic screen further intensified, so that the video pictures of high contrast and low noise component are obtained.
According to the invention, an image intensifier is electro-optically coupled to the luminescent fluoroscopic screen to form an image intensifier unit. It is through this coupled image intensifier that the picture, visible due to activating radiation, is further intensified so that losses such as are due to the optical coupling have no disturbing effect.
The dimensions can be kept relatively small if the image intensifier is a flat image intensifier with near field focussing. A particularly simple design results from the image intensifier unit having as an input screen a first carrier layer to which is applied an input fluoroscopic screen on which a photocathode is vapor-deposited, and there being applied to a second carrier layer, in the direction towards the input, an output fluoroscopic screen covered by a thin conducting film. The fluoroscopic screen can be scanned from the input side if the input fluoroscopic screen consists of thermo-luminescent material and the output fluoroscopic screen of zinc sulfide or cadmium sulfide, and if the first carrier layer is permeable to infrared radiation. One advantageous variant results from scanning taking place on the output side of the image intensifier unit; from the input fluoroscopic screen consisting of cesium iodide and the output fluoroscopic screen of thermoluminescence material; and from the second carrier layer being permeable for infrared radiation and visible light.